Date:         Mon, 17 Nov 2003 01:08:28 -0500
Reply-To:     "Daniel L. Katz" <katzd54@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Daniel L. Katz" <katzd54@YAHOO.COM>
Subject:      Re: Fwd: RE: "Green" solar battery charger
Comments: To: Tom Ring <taring@TARING.ORG>

tom:

in response to your rhetorical question ...

(1) as you well know, an ohm's law device is one in which current is
proportional to voltage: I = V/R. double the voltage and the current
doubles; halve the voltage and the current drops to half; reverse the
voltage and the current changes sign. an ordinary copper wire is a good
example of a practical ohm's law device.

(2) a diode does not obey ohm's law. in particular, reverse the voltage on
an initially forward biased diode and the current stops.

(3) perhaps in the experiment you alluded to current was measured for a
number of different voltages, and effective resistances were computed by
dividing voltage by current for each V,I combination - is it possible that
it was these computed resistances that fit accepted resistance values to
an accuracey of 1%? surely the diode didn't agree with ohm's law to within
1%.

(4) simply diving voltage by current to compute resistance is not an
application of ohm's law. ohm's law is very special, and when a device
obeys ohm's law, it really means that R = constant and the current follows
the straight line relationship, I =V/R. a good analogy might be that we
wouldn't say that a parabala was a straight line because we could compute
the slope of the tangent line at every point on the curve.

(5) no device obeys ohm's law exactly, and ohm's law fails for all devices
at sufficiently high voltages.

dlk





On Fri, 14 Nov 2003 17:01:32 -0600, tom ring <taring@TARING.ORG> wrote:

>Hmm, despite the "failure" of ohms law, we were able to use it in my 500
level
>(that would be graduate level to non engineers) radio frequency non-linear
>transistor design course, we were still able to get answers better that 1
>percent accurate.  Guess we were just lucky.
>
>I'm done with this topic on the list.  Let's take it offline folks.